1. Field of the Invention
This invention pertains to methods and compositions for controlled free radical polymerization in an emulsion or aqueous system using free radicals that provide control under a variety of conditions and with a wide variety of monomers. This invention also pertains to methods for seeding emulsion polymerizations. Furthermore, this invention also pertains to the polymers, copolymers and interpolymers that can be made with such a system.
2. Background Discussion
Controlled free radical polymerizations are generally known, which provide xe2x80x9clivingxe2x80x9d type kinetics. These reactions may proceed by stable free radical or other mechanism. With the stable radical mechanism, it is generally believed that a stable free radical is used to reversibly cap the propagating polymer chain. There are several well-known nitroxide radicals (e.g., nitroxyls) that may be usefully employed in this methodology. See, e.g., U.S. Pat. Nos. 4,581,429, 5,322,912 and 5,401,804, each of which is incorporated herein by reference. The nitroxide radicals disclosed in these references have xcex1-carbon atoms attached to the nitrogen that bear alkyl or aryl moieties (in addition to the oxygen). One of the most common nitroxide radicals is 2,2,6,6-tetramethyl-1-piperidinoxyl radical (TEMPO) and several groups have worked to make this and related radicals commercially viable control agents in a controlled, stable free radical polymerization scheme. See, e.g., WO 98/13392 and WO 98/07758, each of which is incorporated herein by reference. The drawback of TEMPO and related radicals has been the limitation on the monomers that can actually be polymerized. Styrene, substituted styrenes and copolymers including styrene have been prepared, but other desirable, commercially important monomers have not been polymerized in a controlled manner to desired molecular weights. German, et al, xe2x80x9cControlled Radical Polymerization in Emulsion,xe2x80x9d Macromolecule, 1997, 30, 324-326 and Claverie et al, xe2x80x9cNitroxide Mediated Living Radical Polymerization of Styrene in Emulsion,xe2x80x9d Macromolecules, 1998, 31, 4041-4044, both of which are incorporated herein by reference, have investigated TEMPO and other nitroxide radicals in emulsion; however, even in these studies styrene was the only monomer used and the polymerizations proceeded at high temperatures (125xc2x0 C. or higher). TEMPO has proven to be limited in its usefulness. See also WO 98/30601, which is incorporated herein by reference.
Alternative nitroxide radicals were suggested by Grimaldi et al. bearing an electron withdrawing dialkoxyphosphonyl substituent and a hydrogen atom on the xcex1-carbon atoms from the nitrogen. Grimaldi et al. xe2x80x9cSynthesis and Applications to xe2x80x98Livingxe2x80x99 Free Radical Polymerization of a New Class of Nitroxyl Radicals,xe2x80x9d Polymer Preprints, vol. 38, no. 1 (April 1997) and WO 96/24620, both of which is incorporated herein by reference. However, Grimaldi et al. did not focus on water-soluble systems or emulsions. See also EP 0891 986 A1, which is incorporated herein by reference.
More recently, Hawker et al., xe2x80x9cDevelopment of a Universal Alkoxyamine for xe2x80x98Livingxe2x80x99 Free Radical Polymerizations,xe2x80x9d J. Am. Chem. Soc., 1999, 121(16), pp. 3904-3920, which is incorporated herein by reference, discussed the general use of xcex1-hydrido nitroxide radicals. Hawker et al. did not however considered emulsions or water-soluble systems. Also, many nitroxide radicals having a hydrogen atom on the a-carbon (sometimes referred herein to as an xcex1-hydrido nitroxide radical) are known. See, e.g., Janzen et al., J. Am. Chem. Soc., 91:16, pp. 4481-4490 (Jul. 30, 1969); Janzen et al., J. Am. Chem. Soc., 1989, 111, 2206-2070; Janzen et al., J. Am. Chem. Soc., 1986, 108, 6858-6863; and U.S. Pat. No. 3,422,144; each of which is incorporated herein by reference.
The use of water as a dispersing medium or solvent for control free radical polymerization is commercially important for several reasons. First, water is the safest medium from an environmental viewpoint, facilitating the manufacture of consumer products (such as paints or glues). Also, water is one of the least expensive media, providing an economical process. Moreover, the emulsion polymerization process is unique in that aqueous dispersions and emulsion that have direct utility in this for in various applications.
Also, seeded emulsion polymerizations are generally known in order to avoid or simplify the nucleation step. See, e.g., Poehlein et al, xe2x80x9cCharacterization of Water-Soluble Oligomer in Acrylic Acit-Styrene Emulsion Copolymerization,xe2x80x9d J. Appl. Polym. Sci., vol. 50, pp. 2173-2183 (1993) or Ugelstad, et al., xe2x80x9cA kinetic Investigation of the Emulsion Polymerization of Vinyl Chloride,xe2x80x9d J. Polymer Sci., Part C, no. 27, pp. 49-68 (1969), both of which are incorporated herein by reference. Seeded emulsions have also been attempted with TEMPO and TEMPO-like free radicals. See German et al. (cited above). The problem with these attempts is that the systems did not provide particle size control or emulsion quality for commercially important emulsions.
A need exists for a versatile, water-based controlled free radical polymerization process, which can use many types of initiators and can polymerize many types of monomers with a wide variety of process conditions.
This invention provides a method of free radically polymerizing a wide variety of monomers using water-soluble systems or emulsions, and thus provides access to a wide variety of resultant polymers that may be made from such processes. The methods, emulsions and polymers of this invention provide living-type free radical polymerizations, including the ability to re-initiate polymer chains and thus prepare unique polymers, including block copolymers. It is thus an object of this invention to provide a polymerization process that allows access to a wide variety of monomers that may be polymerized alone or together in emulsions or in an aqueous solution system. Moreover, the methods of this invention enable access to a full range of initiators, including fast and water-soluble initiators as well as slow and organic-soluble initiators that might otherwise appear to be less favorable for aqueous-based living-type polymerizations.
These and other benefits can be realized by an emulsion polymerization process that uses water, surfactant, initiator, at least one monomer and a control agent that comprises an xcex1-hydrido nitroxide radical. The control agent can be added to the emulsion as a free radical or as an adduct of the initiator or initiator fragment attached to the control agent. The ratio of control agent to initiator can be in the range of from about 0.01:1 to about 4:1, but is generally most preferably close to 1:1 to provide a commercially reasonable balance between reaction time and living character. The ratio of initiator to monomer is important to the desired molecular weight of the resultant polymer and this ratio can be adjusted to a desired target molecular weight. The control agent may be water soluble in the emulsion process, but may also be relatively water-insoluble.
Another aspect of this invention is an aqueous-solution homogeneous polymerization process having living-type characteristics where water, a water soluble initiator, at least one water soluble monomer and a water soluble control agent that comprises an xcex1-hydrido nitroxide radical are mixed together under polymerization conditions. This system forms water-soluble polymers, including block copolymers. Thus, it is an object of this invention to provide a system for the free radical polymerization of water-soluble polymers in a living type polymerization system using xcex1-hydrido nitroxide radicals.
Yet another aspect of this invention is a seeding process for an emulsion polymerization. First, a fraction of the total monomer that is planned to be added to the polymerization reaction is first mixed with the initiator, control agent (or initiator-control agent adduct), water and surfactant. This combination is mixed and allowed to react for a predetermined period of time under predetermined polymerization conditions. The intent of this first stage is to allow the initiator to form xe2x80x9clivingxe2x80x9d oligomers or seeds with the monomer in the system and the control agent. These are generally referred to herein as xe2x80x9cloaded seeds,xe2x80x9d which are another aspect of this invention. Second and optionally subsequent stages provide for the addition of additional monomer, which can be the same or different from the monomer used in the first stage.
Another aspect of this invention includes the use of lower polymerization temperatures than are traditional with living free radical polymerizations. In some preferred embodiments of this invention, the temperature is about 110xc2x0 C. or less. Another aspect of this invention is block copolymers having blocks with a weight average molecular weight of at least about 25,000.
Other aspects of this invention will be evident to those of skill in the art upon review of this specification, drawings and examples.